* rename setThreadBlockingPolicy to setThreadPolicy (opens the way to
using StrictMode for non-blocking-related things in the future?)
* add allowThreadDiskWrites() and allowThreadDiskReads() to modify the
current policy mask and return the old one. this will allow turning
off part of StrictMode during certain regions of code. (for
instance, writing to disk in Activity onPause...)
Change-Id: Ia1878153713f79299971fdab567fa15b3cb9d56c